1. Field of the Invention
This invention relates to a printing apparatus for printing characters formed by a host computer or the like, or an image read by a scanner, and more particularly, to an image processing apparatus which can be advantageously used when printing dot data by an electrophotographic method, such as a laser-beam printer or the like.
2. Description of the Related Art
When image data read by an image scanner (image reading device) or the like are printed by a laser-beam printer, the procedure of converting the image data into the pixel (picture element) density of the printing apparatus is shown in FIG. 8.
In FIG. 8, the data shown at the left are transmitted from a host computer or the like with a pixel density of 300 dpi (dots per inch), each square representing one pixel. A numeral (for example, 46) indicated in each square, or section, represents the density of that pixel. In the present case, pure black=the highest density is represented by 255, and pure white=the lowest density is represented by 0. A numeral representing a density will be hereinafter termed a gradation number.
The input data received from the scanner or the like via a host computer are first stored in an image memory within the printing apparatus. In order to convert the input data having the pixel density of 300 dpi into a printing pixel density of 150 dpi, pixel density conversion of 4:1 (area ratio) is first performed, as shown in the center of FIG. 8.
In FIG. 8, a numeral 48 indicated in section 211 is the average of the gradation numbers indicated in sections 201, 202, 203 and 204. Subsequently, when performing printing with magnification of 2, data after pixel density conversion are simply magnified by 2.
As described above, in the conventional apparatus, if the pixel density of input data is greater than the printing pixel density of the printing apparatus, the pixel density of input data is converted so as to coincide with the printing pixel density. When magnified printing is performed at the instruction of the operator, printing is performed using image data after the conversion of the pixel density. Hence, the conventional apparatus has the disadvantage that the picture quality of the magnified printed image is inferior to the picture quality of the input data.
In order to solve the above-described problem, a method of performing interpolation using a pixel having a density equal to the average of the densities of adjacent pixels when magnifying image data after resolution conversion may be considered, but deterioration in picture quality, such as undefined edge portions and the like, cannot be avoided.